1. Field of the Invention
This invention relates to electrography, and a method of development, transfer and fixing of dried toner electrographic images. Specifically, it relates to such images for use in overhead projectors, especially to color images for use therein.
2. Description of the Related Art
Electrography refers to the processes of electrophotography, electroradiography, and magnetography. The process of electrography has been described in numerous patents, such as U.S. Pat. Nos. 2,221,776, 2,297,691, and 2,357,809, (Carlson). The process, as taught in these and other patents, essentially comprises production of a latent electrostatic image using photoconductive media and the subsequent development and transfer of a visible image therefrom. A latent electrostatic image may also be formed by spraying the charge onto a suitable charger-retaining surface as taught, for example, in U.S. Pat. Nos. 2,143,214, 3,773,417, and 3,017,560. In magnetography, the latent image is magnetic and may be developed with appropriately magnetized or magnetizable developer particles, as described in U.S. Pat. No. 3,520,811.
Development of the latent image can be accomplished by deposition of developer particles on the electrostatic or magnetic latent image, the most common technique using powder, cascade, or less frequently, liquid developers.
It is well known in the art to use dry powder toner to develop a latent electrostatic image. U.S. Pat. No. 2,855,324 discloses thermoplastic coated receptors to which a dry toner image may be transferred by contact under pressure. U.S. Pat. No. 3,640,749 discloses coating a transferred dry powder image and receptor with a dispersion of a synthetic resin in water. U.S. Patent No. 4,071,362 discloses use of a receptive styrene-type resin on a thermally resistant base film to fuse with thermoplastic coated dry toner particles (i.e., image-fixing is achieved by use of a special toner). U.S. Pat. No. 3,620,726 discloses the use of pigment developer of particle size within the range of 5.0 to 10.0 microns, with not more than 50% of the particles being of less than 1 micron equivalent spherical diameter, thereby reducing background stain. As mentioned, this type of transfer may result in problems of durability.
To avoid such durability problems, various liquid developers have been employed as disclosed in U.S. Pat. 4,337,303, (Sahyun et al.). The liquid toner is encapsulated into a homogeneous continuum of particles within the soft or softened receptor coating. At least 75% of the transferred particles must be embedded within the surface such that they do not protrude.
Particles have also been used in transparencies. U.S. Pat. No. 4,869,955, (Ashcraft et al.) discloses a transparency comprising a polyester support, and at least one toner receptor layer comprising a mixture of an acrylate binder, a polymeric antistatic agent having carboxylic acid groups, a crosslinking agent, and two types of beads, i.e., a butylmethacrylate modified polymethacrylate bead and submicron polyethylene or tetrafluoroethylene beads. The smaller beads are disclosed to improve scratch resistance, and have a particle size of less than one micron, while the polymethacrylate beads are disclosed to assist in transport of the film through the copier and have a particle size of from about 1 to about 5 microns in size.
Where full color images are desired, additional considerations are required. Frequently the prior art processes using dry developing methods showed bright, full color images when the film was inspected, but showed an overall gray tone when the image was projected. As a result the color-tone reproduction range was very narrow.
European patent application no.0349,227, discloses a transparent laminate film for full color image forming comprising two transparent resin layers. The first resin layer is heat-resistant, and the second resin layer must be compatible with a binder resin constituting the toner to be used for color image formation. The second resin layer must have a larger elasticity than that of the binder resin of the toner at a fixing temperature of the toner, preferably in the range of 5 to 1000 times larger than such binder elasticity. While it is stated at page 5, lines 8-26, that resins of the same "kind", i.e., type, e.g., styrene-type or polyester-type, may be used as the toner binder and the second transparent resin layer, the resins must still differ in storage elasticity modulus as previously stated.
It is further specifically stated at page 7, lines 9-14, that where the melt viscosity of the second layer becomes lower than the viscosity of the toner binder resin, it is difficult to develop good color mixing.
It has now been discovered that a good image, even a good full-color image is provided by an electrographic article having a polymeric receptor layer wherein the storage elasticity modulus is equivalent to, or less than that of the toner resin.
It has also been discovered that using polymeric, silica or starch particles in transparent electrographic articles creates a sufficient gap between the film and smooth surfaces with which it contacts that transfer of fuser oil to the projector glass and pooling of fuser oil between the article and a protective sleeve is reduced or eliminated.